This research program is focused on the understanding of neural interactions in the outer plexiform layer of the vertebrate retina. Major effort is directed toward a detailed study of the horizontal cell and its physiology, especially with respect to membrane channels, electrical coupling, voltage dependent conductances, and associated neurons in relation to pharmacological and electrophysiological manipulations. Both intact retinas and dissociated cells maintained in culture are used for these studies. Special attention will be paid to characteristics of turtle horizontal cell response dynamics related to photostimulus modulation which has been shown to have a direct correlation to a model developed using flicker data from human psychophysics. For this purpose, the relatively novel stimulus of a swept frequency sinusoidal stimulus will be used. Effects of surround illumination on recently demonstrated enhancement and linearization of the turtle spot response will also be examined in detail. The all rod retina of the skate will be utilized for comparable studies which will be compared with results from the cone dominant retina of the turtle. This is of special interest with respect to the continued study of feedback between horizontal cells and photoreceptors since it is currently believed that such horizontal cell feedback exists onto cones, not rods so comparisons with results from the skate which has no cones may be especially instructive and provide a good control for evaluation of presumed feedback effects. Studies to further understand the source of the clinically significant b-wave and slow PIII components of the electroretinogram (ERG) are being continued on the skate where results obtained using various pharmacological agents and recording with both conventional and potassium-selective electrods support the view that slow PIII is a potassium dependent response, but are not consistent with the popular notion that slow PIII oiginates across the Muller cell membrane. Using this approach, findings will be related to human psychophysics as well as interpretation of the ERG which is an important clinical tool.